1. Field of the Invention
The present invention relates to an image recording method and an image recording apparatus for performing recording using a dot pattern where dot information is stored, a method for controlling the image recording apparatus, a storage medium storing a program capable of being read by a computer, and an image processing method.
2. Description of the Related Art
There has been a method called a density pattern method, as a method of pseudo-halftone processing for representing gradation by converting a multiple-valued image into a binary image. In this method, in order to represent continuous gradation in an image recording apparatus which can represent each pixel of an image only by binary values, a plurality of binary pixels are collected to form a block, and gradation is represented by a group of such pixel blocks. More specifically, dots are arranged within a dot-matrix cell comprising, for example, 2×2 or 4×4 pixels, and one pixel is represented by obtaining pseudo-gradation with an area ratio. The density pattern method has problems in that the resolution is degraded when the block size is increased, and the number of gradation levels is small when the block size is reduced, resulting in degradation in the degree of gradation because smooth gradation representation is difficult to obtain.
In a method for solving the problems of the density pattern method, pseudo-continuous gradation is represented by preserving an error between a gradation value represented by a pixel block and a gradation value of an input pixel and adding the error to an un-input pixel value. This method is called an error diffusion method. According to this method, it is possible to perform image recording in which resolution and gradation are compatible.
In the above-described conventional approaches, however, a dot pattern having a fixed arrangement of dots is used for each gradation value. For example, in the case of gradation value “1”, as shown in FIG. 41, a dot pattern having a dot at an upper right corner thereof in a 2×2 dot matrix is repeated. By repeating such a regular dot pattern, unevenness in the density having predetermined periodicity is generated in the recorded image.
Such unevenness in the density is due to deviations in the characteristics of respective discharging ports, because the shapes of respective discharging ports slightly differ, the performance of the source for generating an ink discharging force differs depending on the nozzle, and the amount of ink discharge differs at each discharging port due, for example, to a difference in the temperature of each nozzle. Accordingly, the characteristics of each nozzle differ.
If recording is performed using a dot pattern having a fixed arrangement of dots for each gradation level as in the above-described case, the recorded image tends to be influenced by variations in the characteristics of each nozzle, and unevenness in the density and unevenness due to stripes occur in the image, thereby greatly degrading the quality of the image.
In order to prevent the occurrence of unevenness in the density and unevenness due to stripes, (i) in conventional ink-jet printers, multi-path recording control is adopted, in which a recording head scans (passes) the same region on a recording medium a plurality of times using different nozzles of the recording head at respective paths. However, if the pattern of dots to be output is a fixed dot pattern as described above, it is impossible, in some cases, to sufficiently prevent degradation of the quality of the image even by the multi-path recording control.
(ii) In another approach, in order to solve the problems in the above-described approach (i), dot-pattern tables, each storing a plurality of dot patterns, whose number is the same as the number of gradation levels are provided. In this approach, however, the number of necessary dot-pattern tables increases as the number of gradation levels increases, thereby increasing the amount of data of the dot-pattern tables. An increase in the amount of data requires a large-capacity memory, thereby increasing the cost of the recording apparatus.
(iii) In still another approach for solving the problems in the above-described approach (i), the size of the dot-pattern table is increased. This approach, however, requires a large-capacity memory. If the size of the dot-pattern table is simply reduced in order to solve this problem, stripes and unevenness are pronounced in the recorded image, thereby degrading the quality of the image.